Primary and metastatic brain tumors occur more commonly in males than females. This is true regardless of age or region of the world. Thus, sex must affect fundamental mechanisms of neoplastic growth in the central nervous system. In preliminary studies we established conditions that result in the transformation of male but not female astrocytes. This experimental paradigm is an unprecedented opportunity to define the molecular basis for the effects of sex on tumorigenesis. We found that sex affects global gene expression, activation of the tumor suppressor p21cip1, induction of a stem cell phenotype, growth kinetics, cellular senescence and patterns of intracranial malignant growth. Moreover, we found sex differences in human GBM stem cells. In this proposal we will exploit these murine and human GBM models to define the molecular basis for the effect of sex on brain tumorigenesis. In Specific Aim 1 we build on preliminary findings that sexually dimorphic expression of the growth inhibitory molecule p202 results in differential induction of the tumor suppressor p21cip1 and thereby blocks transformation in female astrocytes. In Specific Aim 2 we will follow up on preliminary studies that indicate stem cells are more easily induced in male astrocytes and define how sex, multiple initiating oncogenic events, X chromosome reactivation and cell of origin interact to regulate transformation. In Specific Aim 3 we perform validation studies with human male and female GBM stem cell cultures and test the hypothesis that sex differences in GBM stem cells will impact on responses to therapeutic targeting of CXCR4. Together these studies will advance of fundamental knowledge of cancer biology and have the potential to impact on our approach to treating patients with malignant brain tumors.